Masters Thesis

A compositional, diagenetic, and statistical analysis of marine forearc and reararc basin volcaniclastic sandstones from upper oligocene odp leg 126 and iodp expedition 351 cores, izu-bonin-mariana arc system

ABSTRACT A COMPOSITIONAL, DIAGENETIC, AND STATISTICAL ANALYSIS OF MARINE FOREARC AND REARARC BASIN VOLCANICLASTIC SANDSTONES FROM UPPER OLIGOCENE ODP LEG 126 AND IODP EXPEDITION 351 CORES, IZU-BONIN-MARIANA ARC SYSTEM By Ian Ruttenberg Master of Science in Geology Integrated Ocean Discovery Program (IODP) Expedition 351 and Ocean Drilling Program (ODP) Leg 126 recovered cores of upper Oligocene volcaniclastic rocks (largely marine gravity flow deposits) in the Izu-Bonin Mariana (IBM) Arc system from four drill sites, three in the forearc (Holes 787B, 792E, and 793B), and one in the reararc (Holes U1438B/D). These provide a volcaniclastic record of the mature phase of the IBM arc through IBM arc rifting and initiation of the Shikoku backarc basin. They also record post-depositional alteration processes including authigenic minerals and porewater samples that were squeezed from cores throughout these intervals. Thin sections were prepared from 39 volcaniclastic sandstone samples first impregnated with blue-dyed epoxy for porosity recognition: nine from Hole 792E (438-781 meters below seafloor [mbsf]), ten from Hole 793B (970-1369 mbsf), and 20 from Holes U1438B/D (165-493 mbsf). Petrographic point-count data, ~400 points per sample, were collected using the Gazzi-Dickinson point-count method after thin sections were stained for calcium and potassium silicate phases (feldspar and zeolites). Counted categories (68 overall) included various unaltered to partly altered mineral (e.g., plagioclase, pyroxene, quartz) and glassy (vitric, pumice) to microcrystalline (microlitic, lathwork) volcanic components, authigenic phases (e.g., zeolites and clay minerals), matrix, and primary and secondary porosity types. Data sets were combined with a previously collected data set from Hole 787B (n=29; 119-310 mbsf) (Marsaglia, unpublished). Samples vary in their degree of alteration to authigenic minerals with total zeolite content ranging up to 50%, clay mineral cement up to 60%, and residual primary porosity ranging up to 22%. Most samples show common alteration including pervasive cementation of primary and secondary pores, resulting in minor (0-2%) preservation of secondary porosity. Altered minerals and glass can often be identified by unaltered remnants and altered volcanic lithics can be identified by their texture even when replaced (e.g., vesicularity, degree of crystalinity). Some trends are apparent from petrographic analysis of the samples. Mean QFL values generally plot in the Undissected and Intraoceanic Arc provenance fields. However, samples from Hole U1438B extend into the Undissected Continental Arc provenance field, reflecting potential maturation of the arc up to rifting. Furthermore, Site 787 and U1438 samples are more felsic in composition and Site 792 and 793 samples are more mafic to intermediate in composition, suggesting that each group had a different volcanic provenance. Felsic compositional indicators include colorless volcanic glass and pumice fragments, higher vitric volcanic fragment abundances, and traces of quartz. Mafic to intermediate compositional indicators include brown to black volcanic glass fragments, higher microlitic to lathwork volcanic fragment abundances and pyroxene. Correlation coefficient-based statistical analysis reveals that at each site and across all sites, original composition, porosity and authigenic phases, presence of component types, mean grain size, depth, and porewater geochemistry are weakly correlated to each other, with correlation coefficients (cc) between -0.6-0.6, but have standard deviations too large to be statistically significant. However, some variables do correlate with each other across specific parameters. Although related parameters are the most correlative, some independent correlations are apparent among individual independent parameters of original composition, porosity and authigenic phases, and presence of component types. For example, total authigenic clay is directly proportional to the presence (or absence) of zeolites for all samples (cc=0.83). Mean grain size is weakly correlative to all parameters, but numerous depth correlations were noted. Porewater geochemistry parameters (such as individual cations, pH, alkalinity, etc.) have extensive correlations to other compositional parameters across both the Leg 126 forearc (787, 792, and 793) and Expedition 351 reararc (U1438) sites. For example, Leg 126 porewater salinity is directly proportional to extended (including all intraparticle sources) total carbonate (cc=0.98), and Site U1438 porewater Mg2+ is inversely proportional to total authigenic clay (cc=-0.89). In individual holes, background lithology may be related to sandstone composition (component types), porosity and authigenic phases, mean sandstone grain size, and porewater geochemistry. For example, unusually elevated or depleted abundances of certain parameters were found in stratigraphic sections dominated by shale-containing vitric volcaniclastic or conglomerate lithologies. In a broader sense, the forearc and reararc sites show evidence of having slightly different hydrologic systems. Site U1438, with its gradually changing porewater profiles through the sedimentary column, is located in a clearly open hydrologic system. The Leg 126 forearc sites (787, 792, and 793) have the rapidly-changing and variable porewater profiles expected of a closed hydrologic system, suggesting that the porewater chemistry is not being equally equilibrated across the entire sedimentary column. At these forearc sites, there are strong compositional correlations to porewater geochemical parameters.

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